A turning propeller shaft on a ship becomes electrically insulated from the hull by the lubricating oil film in the bearings and by the use of non-metallic bearing materials in the tail shaft. When the shaft is insulated in this way an electrical potential can be measured between the shaft and the hull and this can accelerate corrosion in the ship.
If the ship has a system of cathodic protection, whether it is sacrificial anode or an impressed current system, the shaft insulation will prevent the propeller and the boss from receiving protection.

The electrical potential between the shaft and the hull can also cause a heavy current to flow in bearings when the oil film breaks down or is contaminated with seawater. This current can cause deep pitting of the bearing surface.? Excessive wear on the shaft bearings can often be traced to this cause.
Now in addition it’s necessary to reduce the spark erosion causing the excessive wear on main engine metal bearings and this shaft earthing is the most appropriate method.
All the troubles can be avoided and cathodic protection can be extended to the propeller if the shaft is properly earthed with a propeller shaft slip ring. The effectiveness of the shaft earthing system should ensure a maximum contact resistance of no greater than 0.001 ohms for a water filled bearing and 0.01 ohms for an oil filled bearing on millivolt meter.

Our own tests indicate that high silver content brushes running on a silver track have repeatable low conductivity that can maintain these limits and ensure a low resistance contact is maintained even under dirty conditions.
The shaft earthing assembly comprises a pair of high silver content / graphite compound brushes mounted in balanced brush holder, running on a copper slip ring with solid silver inlay track.
Each brush holder has a adjustable spring tensioner which is supplied preset to the minimum, and results in a pressure of 225g/cm2.
With this arrangment our high quality brush is in excess of one year life.

Brushes for shaft earthing device get worn down y meshanic friction on slip ring for the drain of spark inside main engine and hih elctric potential from stern bearing/propeller. Thereadings of earthing are checked through mV meter shich should be maintained below a level.